1. Field of the Invention
The present invention relates to a direct-drive wind turbine generator and bearing structure suitable for the direct-drive wind turbine generator, and in particular, relates to a structure for supporting a main shaft and a generator in a direct-drive wind turbine generator.
2. Description of the Related Art
One known form of wind turbine generators is the direct-drive wind turbine generator. In a direct-drive wind turbine generator, a wind turbine rotor and a generator are directly connected by a main shaft, while the rotation of the wind turbine rotor is transmitted to the electric generator by a speed-up gear with the number of rotations increased in a geared wind turbine generator.
A direct-drive wind turbine generator requires special, consideration in designing the structure for supporting the main shaft and the generator, because the size of the generator is large due to the use of a synchronous generator, and the generator and the main shaft are directly connected. In general, the main shaft is rotatably supported with two bearings, and a structure for preventing rotations of the stator casing of the generator is provided. Hereinafter, the structure for preventing rotations of the stator casing of the generator is referred to as torque support. Due to rotations of the main shaft, a torque is applied to the stator casing of the generator in the circumferential direction of the main shaft. It is a role of the torque support to support the stator casing so that that the stator casing does not rotate even when a torque is applied. One or two generator bearings may be additionally provided between the main shaft and the stator casing to support the stator casing thereby. A structure for rotatably supporting the main shaft with two bearings and supporting the stator casing with a torque support is disclosed in European Patent Application No. EP1327073 B1 (Patent Document 1), European Patent Application No. EP2014917 A1 (Patent Document 2) and corresponding Japanese Patent Application Publication P2009-019625A (Patent Document 3), and International Publication WO2007/111425 (Patent Document 4), for example.
Here, bearings with the aligning capability (bearings that allows flexing and tilting of the shaft) are used in general, as bearings which support the main shaft of a wind turbine generator. This is considered to be based on a technical idea that flexing of the main shaft is generated in a direct-drive wind turbine generator and the flexing needs to be absorbed. For example, EP1327073 B1 discloses that bearings for supporting the main shaft allow flexing of the main shaft (e.g. Claim 1). Additionally, International Publication WO2007/111425 discloses that a toroidal roller bearing is employed as a bearing near the rotor head and that a spherical roller bearing is employed as a bearing near the generator, hence compensating the misalignment and tilting of the main shaft.
According to a study by the inventor of the present invention, however, the structure which supports a main shaft with two bearings having the aligning capability and further supports the stator casing with a torque support is not appropriate in order to keep the gap between the stator and rotor constant. FIG. 8 shows the reason. Considered in the following description is a structure which supports a main shaft 103 with first and second bearings 101 and 102, and supports the torque working on a stator casing 106 of a generator 105 in the circumferential direction of the main shaft with a torque support 104, as shown in FIG. 3. Here, l1 is the distance between the load point on the side of the rotor head and the first bearing 101; l2 is the distance between the first bearing 101 and the second bearing 102; and l3 is the distance between the second bearing 102 and a point at which force works from the torque support 104 to the stator casing 106. Additionally, R1 and R2 are the support reaction forces applied by the first bearing 101 and the second bearing 102, and R3 is the support reaction force applied to the stator casing 106 by the torque support 104.
When the two bearings (the first bearing 101 and the second bearing 102) for supporting the main shaft 103 both has an aligning capability, angles of flexure γ1 and γ2 are caused at the respective positions thereof. Due to the angle of flexure γ2 and the distance I3, which is inevitably present because of the layout of the wind turbine generator, the support reaction force R3 is caused even when no torque is worked on the torque support 104. Here, the magnitude of the support reaction force R3 is the product of the spring constant of the torque support 104 times the strain δ.
The support reaction force R3 is not preferable, since an unbalance of the gap between the stator and rotor of the generator 105 is caused. When a permanent magnet synchronous generator (PMSG) is used as the generator 105, in particular, the problem of the unbalance of the gap is significant. In detail, a permanent magnet synchronous generator (PMSG), in which magnetic attractive forces of field magnets and various electric forces work, requires surely keeping the gap between the stator and rotor and reducing various vibration mode displacements. Due to the support reaction force R3, however, the stator casing 106 is displaced correspondingly to the internal clearance of the generator bearing and the stator casing 106 itself is slightly deformed. As a result of the displacement corresponding to the internal clearance and the deformation, an unbalance of the gap between the stator and rotor is caused, and mode vibration due to bending is caused in addition to the magnetic vibration caused by the rotation. Occurrence of a bending mode vibration is not preferable in view of an increase in the vibration of the wind turbine generator. Additionally, occurrence of bending mode vibration increases fatigue loads and causes a problem that structural members (e.g. the main shaft 103, the torque support 104, and the stator casing 106) need to be designed to have a high strength, resulting in the increase of the weight.